Electro-tactic ionic liquid droplets
Francis, Wayne and Wagner, Klaudia and Beirne, Stephen and Officer, David and Wallace, Gordon and Florea, Larisa and Diamond, Dermot (2015) Electro-tactic ionic liquid droplets. In: MicroTAS 2015, 25-29 Nov 2015, Gyeongju Korea.
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Here we report for the first time electro-guided, self-propelled droplets, which are composed solely of an ionic liquid (IL), namely trihexyl(tetradecyl)phosphonium chloride ([P6,6,6,14][Cl]). These self-propelled droplets travel along an aqueous-air boundary and are guided to specific destinations within the fluidic network through the use of electro-chemically generated Cl- gradients. The direction of movement can be controlled by switching the impressed voltage (9V, ON or OFF) and polarity of the electrodes in contact with the electrolyte solution.
Controlled release of surfactants has been investigated previously as a method of controlling surface tension in aqueous systems in order to achieve spontaneous movement of droplets at the air-liquid interface [1,2]. When a surfactant is released into an aqueous solution, the surface tension is lowered. Liquid flows from areas of low surface tension to areas of high surface tension, a phenomenon known as the Marangoni effect. Using stimuli-responsive surfactants, smart droplets have been designed which can solve complex mazes  or can be attracted or repelled by light , in a contactless manner.
Electro-tactic movement of the droplets is due to the controlled release of the [P6,6,6,14]+, a very efficient cationic surfactant, which is a constituent of the IL droplet (Fig. 1). The asymmetric release of the cationic surfactant is controlled through modulation of the IL counter anion (Cl-) solubility, as this controls the rate of release of the cationic surfactant in order to maintain electroneutrality within the droplet. The solubility of the [P6,6,6,14]+ is limited in aqueous solutions and is dependent on the local ionic strength of the solution (Fig. 2). Therefore in ionic strength gradients there is a differential release of the surfactant from droplet boundary into the solution, which in turn results in an asymmetrical surface tension gradient around the droplet. This leads to Marangoni like flows, which propel the droplet from areas of low surface tension to areas of high surface tension.
The chip used in this work was 3D printed (Objet350 Connex printer) as were the titanium mesh electrodes (Realizer SLM-50) embedded in the chip. By applying an external electric field to the solution, a [P6,6,6,14][Cl] droplet can be moved from the cathode (-) to the anode (+) (Fig. 3). The external electric field causes migration of ions, which results in concentration enrichment of ions in the proximity of the electrodes , Na+ at the cathode (starting position) and Cl- at the anode (destination). The resulting ion migration towards the electrodes creates an ionic strength gradient within the channel which controls the movement of the droplet. Additionally, the applied electric field causes Faradic arrangement of the charged ions within the IL droplet . This also creates concentration gradients of the ions within the droplet, reinforcing the droplet movement mechanism. The electro-generation of gradients therefore provides a simple means to control the speed and direction of movement of droplets within fluidic channels in a very flexible manner.
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